The present invention relates to an apparatus for ion implantation that is improved in the mechanism for supporting and cooling wafer disks.
FIG. 1 shows a cross section of a wafer disk with its support that is to be mounted in a conventional apparatus for implanting ions into semiconductor wafers by a batch process and which is equipped with a cooling means that employs a coolant. The periphery of the wafer disk 1 is inclined to form part of a conical surface and a plurality of recesses are formed in that peripheral portion, with the bottom of each recess proving a circular face 3 for receiving a wafer 2. The rotating shaft 4 of the wafer disk 1 is supported axially by a bearing 5 which, in turn, is fitted on a tubular member 6. As the wafer disk 1 rotates at ion implantation, the wafers 2 are held in intimate contact with the associated faces 3 by centrifugal force while an ion beam strikes the wafer disk 1 in a predetermined direction to implant ions into every part of each wafer by rotating the disk 1 and scanning it in the direction of arrow a. In order to prevent the wafers from becoming hot during ion implantation, a passage 7 for coolant is provided in the wafer disk 1, and coolant supply channels 8 and discharge channels 9 that communicate with the passage 7 are connected to a coolant supply source (not shown) via a bearing and coolant introducing rotary seal mechanism 12 having annular channels 10 and O-ring seals 11 provided for the tubular member 6, so that a coolant is supplied through the passage 7 to cool the wafer disk 1.
As described above, the rotating shaft 4 of the wafer disk 1 is supported axially by the bearing 5. Since this bearing mode involves mechanical contact, it requires periodic maintenance (parts replacement) and is not applicable to the case where the wafer disk 1 must be rotated at a speed higher than a certain level. This is also true with a mode of cooling the wafer disk using a fluid introducing rotary seal mechanism which similarly has a mechanical contact area. In addition, with the recent demand for keeping the high quality wafers after ion implantation, a need has arisen to implant ions in a controlled accurate dose. To meet this need; an ion beam current, or the current that flows through the wafer disk 1 as a result of ion implantation is measured with an ammeter 13 that is connected between the tubular member 6 of the bearing and fluid introducing rotary seal mechanism 12 and the ground (one end of the power supply for an ion beam generator not shown is grounded). However, triboelectricity will develop as a result of contact with the bearing mechanism and the current due to the charge buildup will also flow into the ammeter 13. If the ion beam current is on the order exceeding mA (milliamperes), the current due to the triboelectricity can be neglected. However, with a high-energy beam such as an ion beam under an acceleration voltage on the order of MV (megavolts), the amount of ion current flowing will not exceed about 100 .mu.A and the current due to the triboelectricity is large enough to become a current noise for the ion beam current of interest, lowering the precision of measurement of a small beam current that flows as a result of low-dose ion implantation. The triboelectricity under consideration also develops by the flow of the coolant fluid which is used to cool the wafer disk 1 and by the contact thereof with a certain component in the fluid introducing rotary seal mechanism, and irrespective of its origin, the triboelectricity lowers the precision of measurement of the ion beam current.
FIG. 2 shows a cross section of a wafer disk that is to be mounted in another conventional apparatus for implanting ions into semiconductor wafers by a batch process and which is equipped with a cooling means that employs a coolant, in which the same reference numerals as those in FIG. 1 denote the same or like parts. This apparatus is substantially the same as that shown in FIG. 1 except that a wafer disk 1 is formed as a flat plate and wafers 2 are to be retained on receiving faces 3 that are defined by the bottom surfaces of inclined recesses formed in the periphery of the disk 1. Thus, detailed description of this apparatus will be omitted.